1. Field of the Invention
The present invention relates to the reduction of the required amount of signal processing in a modulator/demodulator (modem) which is transferring packet-based data or other information which is intermittent in nature on a communication channel.
2. Related Art
Modern data networks commonly use complex digital signal processing (DSP) devices called modems to transport data over communication channels. Data is typically transported via an analog transmission signal which is representative of a synchronous, constant rate bit stream. This form of communication channel is suitable for the transmission of real-time information such as voice or video. However, it is increasingly common to use modems for the transmission of packet-based information. For example, packet-based information is used to access the Internet and the World Wide Web. However, packet-based information is typically bursty in nature, with an average data rate which is often much less than the available peak data transfer rate of the communication channel.
FIG. 1 is a block diagram of a transmitter circuit 100 of a conventional modem. Transmitter circuit 100 includes packet queue 101, framer 102, channel coding circuit 103, output shaper 104, modulator 105 and digital-to-analog (D/A) converter 106. In accordance with conventional modem protocols, transmitter circuit 100 transforms source data received by packet queue 101 into a continuous time analog transmit signal, which is provided at the output terminal of D/A converter 106.
More specifically, within transmitter circuit 100, the source data is grouped into packets and stored in packet queue 101. These packets are not synchronous with respect to the modem bit clock, but arrive at packet queue 101 at random times. Framer 102 receives the packets from packet queue 101, and in response, composes a continuous bit stream which is synchronous with respect to the modem bit clock. To create such a synchronous bit stream in response to the asynchronous packets, framer 102 generates idle information (i.e., nulls or a marking tone) when no packets are available, and generates packet data when packets are available. The packet data and idle information are delineated in such a way that a receiver circuit of a modem (see, e.g., FIG. 2) can determine where the packet boundaries lie.
The synchronous bit stream generated by framer 102 is then coded by channel coding circuit 103. Channel coding circuit 103 is used to compensate for noise and distortion in the communication channel. Channel coding circuit 103 provides redundant information (e.g., convolutional encoding) to allow for error correction. Channel coding circuit 103 further performs a scrambling function, as well as mapping the coded bit stream onto symbol values. The stream of symbol values generated by channel coding circuit 103 is provided to output shaper 104.
Output shaper 104 digitally filters the stream of symbol values received from channel coding circuit 103. Output shaper circuit 104 limits the frequency bandwidth of these symbol values within a predetermined range and may also be adjusted to help compensate for channel distortion. The filtered sample stream provided by output shaper 104 is provided to modulator 105, which modulates a carrier signal by the filtered sample stream. The output of modulator 105 is provided to D/A converter 106, which generates an analog TRANSMIT signal for transmission on the communication channel (i.e., telephone line).
Transmitter circuit 100 exhibits three distinct disadvantages. First, because transmitter circuit 100 transmits constantly (either packet data or idle information), a modem can be functionally connected to only one telephone line at any given time. Moreover, only a small percentage of the total information carrying capacity of the communication channel is used to transmit data, while a large percentage of this capacity is used to transmit idle information. Additionally, transmitter circuit 100 is unsuited to multi-drop operation on a single communication channel. The first disadvantage mentioned above is particularly deleterious where a number of xDSL modems are collected together in a central office to provide data communications to a number of remote locations. In this case, each remote location requires a dedicated xDSL modem in the central office.
The analog TRANSMIT signal is transmitted over the telephone line to the telephone company central office. Within the central office, an analog to digital converter converts the analog TRANSMIT signal into a digital signal. This digital signal is multiplexed onto a digital backbone circuit and routed to a second central office location. The digital signal is demultiplexed within the second central office location and routed over a digital trunk to a digital server which performs additional processing on the digital signal.
FIG. 2 is a block diagram of a receiver circuit 200 of a conventional modem. Receiver circuit 200 includes analog-to-digital (A/D) converter 201, resampler 202, equalizer 203, carrier recovery circuit 204, symbol decision circuit 205, channel decoding circuit 206, framer 207, packet queue 208, echo canceler 209, timing update circuit 210, equalizer update circuit 211 and carrier update circuit 212. Carrier recovery circuit 204 and symbol decision circuit 205 are sometimes referred to as a demodulator circuit. A/D converter 201 is coupled to the telephone line to receive the analog signal from the telephone company central office. A/D converter 201 samples this analog signal, thereby converting the analog signal into a digital signal.
The modem which includes receiver circuit 200 also includes a transmitter circuit (i.e., a near end transmitter circuit, not shown) which is similar to transmitter circuit 100. During full duplex operation, this near end transmitter circuit may be generating a TRANSMIT signal at the same time that receiver circuit 200 is attempting to receive the analog signal from the remote (or far end) transmitter circuit 100. Under these conditions, receiver circuit 200 may receive an echo of the TRANSMIT signal. Echo canceler 209 generates a signal which is a replica of this echo. The signal generated by echo canceler 209 is then subtracted from the output signal provided by A/D converter 201.
Resampler 202 adjusts the raw input samples received from A/D converter 201 to match the symbol rate of the transmitter circuit 100. Timing update circuit 211 extracts timing information which is used to control resampler 202. Equalizer 203 compensates for linear distortions introduced by the communication channel (e.g., the telephone line). Carrier recovery circuit 204 extracts the carrier signal from the received signal and provides rough symbols (or a soft symbol decision) to symbol decision circuit 205. Symbol decision circuit 205 quantizes the rough symbols and makes hard decisions as to the identity of the received symbols. Equalizer update circuit 211 and carrier update circuit 212 receive the symbols provided by symbol decision circuit 205. In response, equalizer update circuit 211 and carrier update circuit 212 determine quantizer error. In response to this quantizer error, equalizer update circuit 211 and carrier update circuit 212 adjust the coefficients used by equalizer 203 and carrier recovery circuit 204, respectively, thereby improving the accuracy of subsequent hard symbol decisions.
Channel decoding circuit 206 uses redundant information present in the received analog signal to correct for quantizer errors. Channel decoding circuit 206 typically implements a maximum likelihood sequence estimator (MLSE) circuit (such as a Viterbi decoder or other form of error correction. Channel decoding circuit 206 provides a decoded bit stream to framer 207. Finally, framer 207 decodes the bit stream into packet data, discarding the idle information, and loading the packets of data into packet queue 208.
The operation of receiver circuit 200 is significantly more complex than the operation of transmitter circuit 100. Substantial signal processing is performed by receiver circuit 200, typically many hundreds or thousands of operations per symbol processed. Much of the signal processing is concentrated in equalizer 203, echo canceler 209, and channel decoding circuit 206. A significant percentage of this signal processing is dedicated to the processing of the idle information generated by transmitter circuit 100.
It would therefore be desirable to have a modem system which is capable of utilizing a greater percentage of the information carrying capacity of the telephone line to transfer packet based data. It would also be desirable to have a modem system which minimizes the signal processing which must be dedicated to the processing of idle symbols. It would further be desirable to have a modem system which enables a common modem to be functionally connected to a plurality of telephone lines at the same time. It would further be desirable to have a modem system which enables a common telephone line to be used with a plurality of modems in a multi-drop configuration.
Accordingly, the present invention provides a method for operating a modem on a communication channel which includes the following steps. A receiver circuit of the modem is coupled to receive a continuous analog signal which is transmitted on the communication channel. This continuous analog signal includes both packet information and idle information. The receiver circuit monitors the analog signal to detect the presence of the idle information. Upon detecting the presence of the idle information, the receiver enters a standby mode. In the standby mode, the amount of processing performed by the receiver circuit is reduced.
The reduction of the amount of processing performed by the receiver circuit can be achieved by disabling and/or reducing the processing precision of selected elements within the receiver circuit. For example, a symbol decision circuit, a channel decoder and a framer within the receiver circuit can be disabled during the standby mode in one embodiment of the invention. Moreover, the processing precision of other elements, such as an echo canceler, update circuits and an equalizer can be reduced when the receiver circuit is in the standby mode.
To detect the presence of the idle information, the receiver circuit fully demodulates the analog signal to provide a digital bit stream. This digital bit stream is processed by the receiver circuit to determine when packet data ceases to be transmitted on the communication channel, and the transmission of idle information commences. At some point after the receiver circuit detects the start of the idle information, the receiver circuit enters the standby mode. At this time, various elements within the receiver circuit are disabled and/or operated with reduced precision. In addition, an idle bit pattern, which is synchronous with the idle bit pattern generated by the associated transmitter circuit, is converted to a plurality of expected idle symbols. The expected idle symbols are then compared with a plurality of soft symbols which are generated by the receiver circuit in response to the analog signal using reduced processing within the receiver circuit. The receiver circuit remains in the standby mode as long as the expected idle symbols match the soft symbols.
The receiver circuit can further store a most recent history of the analog signal in a buffer. After the standby mode is exited, this buffer can be accessed, thereby enabling the receiver circuit to re-process the most recent history of the analog signal. This helps ensure that no packet information is lost due to the inherent delay in detecting the presence of packet information.
In accordance with another aspect of the present invention, the receiver circuit can monitor the quality of the analog signal on the communication channel and reduce the amount of processing performed by the receiver circuit if the channel quality exceeds a predetermined level. This further reduces the processing requirements of the receiver circuit.
In accordance with another embodiment of the invention, a burst mode protocol is provided for operating a modem on a telephone line. The burst mode protocol involves modulating packets of digital information by a transmitter circuit of the modem, wherein the packets of digital information are converted into analog signal bursts of discrete duration. These analog signal bursts are transmitted from the transmitter circuit to the telephone line. However, no signal is provided from the transmitter circuit to the telephone line between the analog signal bursts. In one embodiment, a non-idle state signal is appended to the beginning of the analog signal bursts by the transmitter circuit, thereby signalling the presence of the analog signal bursts.
A receiver circuit of the modem monitors the telephone line to detect the presence and absence of the analog signal bursts. This monitoring step is performed by a non-idle detector within the receiver circuit. When the non-idle detector detects the presence of the analog signal bursts on the telephone line, the non-idle detector causes the receiver circuit to demodulate the analog signal bursts using full processing capabilities of the receiver circuit. However, when the non-idle detector detects the absence of the analog signal bursts on the telephone line, the non-idle detector disables the demodulating function of the receiver circuit. This greatly reduces the processing requirements of the receiver circuit when there are no analog signal bursts present on the telephone line.
In one embodiment, the non-idle detector determines the presence and absence of the analog signal bursts on the telephone line by monitoring the telephone line for the presence and absence of carrier energy. Alternatively, the non-idle detector can monitor the telephone line for the presence and absence of a non-idle state signal provided by the transmitter circuit.
In accordance with the burst mode protocol, there are certain periods during which the transmitter circuit is not transmitting any signals. During these periods, the echo canceler of the associated local receiver circuit can be disabled, since there will be no echo signal to cancel during these periods. This further reduces the processing requirements of the receiver circuit.
In accordance with another aspect of the present invention, the receiver circuit can monitor the quality of the analog signal bursts on the telephone line and reduce the amount of processing performed by the receiver circuit if the line quality exceeds a predetermined level. This further reduces the processing requirements of the receiver circuit.
In accordance with another embodiment of the present invention, a plurality of remote transmitter circuits, which are coupled to separate telephone lines, generate analog signal bursts in accordance with the burst mode protocol. The separate telephone lines are connected together at a central location where the analog signal bursts are multiplexed to a number of receiver circuits. A non-idle detector is coupled to receive the analog signal bursts from each of the transmitter circuits, and to detect the presence and absence of the analog signal bursts on the telephone lines. Typically, only a small number of the telephone lines will be transmitting analog signal bursts at any given time. The analog signal bursts are therefore multiplexed into a number of receiver circuits which is less than the number of telephone lines. That is, each receiver circuit can process analog signal bursts from a plurality of telephone lines. As a result, the number of receiver circuits required to handle information from a given number of telephone lines is advantageously reduced. In a particular embodiment, different sets of update coefficients are enabled within the receiver circuits, depending upon which telephone line is currently coupled to the receiver circuit.
The present invention also includes a method for operating a plurality of modems on a single telephone line (i.e., multi-drop operation). This method includes the steps of (1) modulating packets of digital information by the modems, wherein the packets of digital information are converted into analog signal bursts of discrete duration, (2) transmitting the analog signal bursts from the modems to the telephone line, (3) providing no signal from the modems to the telephone line between the analog signal bursts, and (4) arbitrating the transmitting of the analog signal bursts from the modems to the telephone line such that only one modem is transmitting analog signal bursts to the telephone line at any given time.
In one variation of the multi-drop method, each of the analog signal bursts includes a preamble and a corresponding main body. Each preamble is transmitted in accordance with a predetermined first modem protocol. However, the main bodies can be transmitted in accordance with different modem protocols which are different than the first modem protocol. For example, the different modem protocols may implement different data rates, modulation formats and/or protocol versions. The modem protocol associated with each of the main bodies is identified by information included in the corresponding preamble. This variation enables devices having different operating capabilities (e.g., personal computers and smart appliances) to be operably coupled to the same telephone line in a multi-drop configuration.
The present invention further includes a method for implementing a multi-line network access circuit. In this embodiment, digital data packets are transmitted from a plurality of sources (e.g., ISPs) to a multi-line network circuit. The digital data packets do not include idle information. The multi-line network access circuit identifies the telephone lines associated with the digital data packets using a destination address monitor. Digital data packets from different sources are multiplexed to a common digital signal processing (DSP) resource. This common DSP resource modulates digital data packets from different sources. The multi-line network access circuit then de-multiplexes the modulated digital data packets onto telephone lines corresponding to the destination addresses. In one variation, a common idle generator within the multi-line network access circuit is used to generate common idle information for each of the telephone lines. In another variation, a non-idle state signal generator within the multi-line network access circuit is used to generate non-idle state signalling for each of the telephone lines.
Yet another embodiment of the present invention provides a method of implementing a multi-cast network access circuit. In accordance with this method, a digital data packet is transmitted from a source to the multi-cast network access circuit. In this embodiment, the digital data packet does not include idle information. The digital data packet identifies a plurality of destination addresses to which the digital data packet is to be transmitted. The digital data packet is routed to a digital processing resource and modulated. The modulated digital data packet is de-multiplexed to a plurality of telephone lines which correspond to the destination addresses, thereby completing the multi-cast operation.
The present invention will be more fully understood in view of the following detailed description taken together with the drawings.